This invention relates to fluorescent lamps. More particularly, this invention relates to aperture fluorescent reprographic lamps.
An aperture fluorescent reprographic lamp is a high output or very high output type fluorescent lamp which is designed with a phosphor coating extending part way around the lamp and in such a manner as to leave a slot of clear glass along the length of the lamp. The slot of clear glass may extend the full length of the envelope as shown, for example, in FIG. 1 of U.S. Pat. No. 3,141,990 to J. G. Ray or, alternatively, may extend substantially the full length of the lamp envelope as shown in FIG. 1 of U.S. Pat. No. 3,886,396 to Hammer et al. The purpose of this construction is to concentrate a beam of light through the clear glass section.
Green-emitting zinc orthosilicate phosphor, also known by its mineral name willemite, was one of the earliest phosphors used for reprographic applications and is still used in some of the earlier photocopiers. The major limitation of this phosphor for reprographic applications lies in its severe degradation in emission with time of lamp operation, known as maintenance loss. Hence, alternative phosphors with better maintenance have been developed through the years, such as, for example, cerium terbium coactivated magnesium aluminate phosphor. Such better maintenance phosphors are, however, significantly more expensive than the green-emitting zinc orthosilicate phosphor because they employ costly rare-earth activators.
Efforts to improve the performance of green-emitting zinc orthosilicate phosphor in aperture lamps have included the use of overcoats, interfacial coats between the phosphor and glass, and the inclusion of additives such as Sb.sub.2 O.sub.3 in the phosphor slurry suspension prior to lamp making. None of these modifications have had a major effect on improving lamp performance to the point of making them competitive with the more costly phosphor developed as replacements for willemite. For example, in U.S. Pat. No. 3,141,990 to J. G. Ray, an aluminum oxide interface is used in a reprographic lamp to prevent discoloration of the lamp glass. U.S. Pat. No. 3,599,029 to Martyny discloses the use of a double interfaced layer of TiO.sub.2 and Al.sub.2 O.sub.3 between the phosphor and glass to improve maintenance. Hammer and Kaduk, in U.S. Pat. No. 3,886,396, disclose the use of a 1 mm-particulate overcoat to improve maintenance. Kaduk and Hammer in U.S. Pat. No. 3,995,192, disclose use of an improved reflector layer containing titanium dioxide and up to 15 weight percent magnesium oxide beneath the phosphor. In U.S. Pat. No. 3,995,192 Hammer discloses an reflector layer containing titanium dioxide and up to 15 weight percent alumina.
Because a decrease in light output still occurs during the lifetime of an aperture fluorescent reprographic lamp, there still remains a significant need for additional improvement.